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How To Insulate An Unvented Crawl Space




Dan K. of Toledo, OH wrote:

Good Morning Itchy,

I have a crawl space that's not vented. What's the best way to insulate it?

Insulating a unvented crawl space is actually much easier than a crawl space that's vented. Don't get me wrong, insulating any crawl space is nasty work and you're about to have some memorable fun. No sense in scratching our heads any longer - lets get started.

First you have to dress for the occasion. Put your coveralls on, dust mask and goggles one. Take your drop light and climb down into the crawl space. Remove any junk and debris so you have a open work area.


Using visqueen, about a thickness of six-mil you need to cover the ground with it. Make sure each piece overlaps at least a foot and along the walls it should go up about 6 inches. You might encounter some tight areas in the crawl space where there isn't much clearance. Go ahead and grunt and swear if you want to - nobody can hear you anyway.

Now comes the fun part of insulating unvented crawl spaces. Running parallel to the floor joists are the stringer joists. Place a unfaced insulation batt against the subfloor and staple it into place. Continue this process with all stringer joist areas of the unvented crawl space.


At the end of every floor joist there is a header joist that needs to be insulated. Cut pieces of unfaced fiberglass batts to fit every header joist area snuggly. Continue this process around the entire perimeter of the unvented crawl space.

The sill needs to be insulated. The easiest way to do this is to use furring strips and nail the batts to the edge of the sill plate. Make your cuts so the insulation drapes down the wall about two feet into the crawl space. You should use 2X4's or 2X6's to secure the insulation when it's close to the ground.

Next take a look at your work and fill any gaps with scrap insulation. Like I said, insulating unvented crawl spaces is disgusting work. Hopefully you didn't get any insulation inside of your shirt. If you did you might find yourself sleeping on the couch tonight after your wife kicks you out of bed for scratching too much.

Finally, get out of the crawl space and insulate the access door by stapling a batt to it. Next, get out of those filthy clothes, throw them in the washer and go jump in the shower. You deserve it because you have just accomplished what many professional insulators consider to be their worst nightmare.

Related Tips and Advice:

How To Insulate Your Attic With Fiberglass Batts or Blankets
How To Install Blown In Insulation In Your Attic
How To Insulate A Concrete Wall
How To Insulate Ducts


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